1. Field of the Invention
The present invention relates to power assist steering systems for vehicles and, in particular, to electric power assist steering systems employing torque sensors for vehicles provided with handlebars.
2. Description of the Related Art
Electric power assist steering systems are well-known and are used for such purposes as recreational vehicles and small utility vehicles. One type of electric power steering system includes an electric motor which is coupled to the steering shaft, typically including a worm mounted on the drive shaft of the motor which engages a worm gear mounted on the steering shaft.
In some prior art examples the systems include a hollow steering shaft which has a narrower internal shaft (also known as the torsion bar) serving as a transducer for a torque sensor. Examples are found in U.S. Pat. Nos. 6,360,841; 7,182,169; 7,183,681; and 7,077,235. The steering shaft itself is typically made of several components. The result may be a system which lacks a sharp and direct steering feel due to play between the different components.
These earlier steering devices with torsion bars are typically acceptable for multiple turn steering systems such as cars and trucks. A typical torsion bar in such a system has a ±4° to ±8° sensing range and a steering wheel range of 720° to 1440°. The torsion bar compliance is approximately 1% of the steering range. However, when a steering system of this type is employed in a vehicle with roughly 90° steering range, for example vehicles with handlebars or tiller steering, then the torsion bar compliance can be 9°-18° due to the higher torque applied to the steering shaft of such vehicles.
Referring to stiffness, torsion bars employed in the past typically range from 120 Nm/rad. to 900 Nm/rad. The electric power steering shaft stiffness is dominated by the torsional region the steering shaft as referenced in SAE paper 2006-01-1320. Such systems use a relatively soft shaft (in torsional stiffness) because of the sensing technology employed. The more displacement, the higher the sensitivity that can be achieved with displacement sensors such as a potentiometer.
Another example of using torsion bars with magnetoelastic sensing technology can be found in U.S. Pat. No. 6,360,841 with ±4 or ±8° of compliance. The stiffness is dominated by the torsional region. A low stiffness system makes it difficult however for handlebar vehicles to avoid oscillation during double lane change maneuvers if they were equipped with such systems.
For instance, when a torsion bar of this type is employed in such a vehicle in a double lane change driving test, the steering system may experience significant overshoot and oscillation. Accordingly there is a need for an improved electric power assist steering system suitable for such purposes as recreational vehicles, particularly those employing handlebars for steering.